The present invention relates to a method for treating an aqueous waste to remove dispersed and dissolved solids. It relates particularly to a treatment of aqueous waste from a textile desizing operation which permits recovery of a desired polymeric size as a reusable material.
In the textile industry, water-dispersible sizing agents such as polyvinyl alcohol, starch, carboxymethylcellulose, other polymeric materials and their mixtures are coated on fibers to protect them during weaving or knitting and to improve processing operations. These sizes are later removed by hot water treatment and the dilute aqueous waste streams thereby produced are discharged to waste treatment systems or directly to the environment. When a synthetic organic polymeric size such as polyvinyl alcohol (PVA), carboxymethylcellulose (CMC), or other such material is involved, the cost of the size as well as increasing restrictions against pollution of the environment makes necessary some kind of treatment of the aqueous waste stream, not only to remove the dispersed size to as great an extent as practical, but also to recover it as a reusable material.
Ultrafiltration has been used to recover PVA from such wastes but this process yields a concentrate containing only about 8-10 percent PVA and the process is not applicable to commonly used mixtures of PVA with starch. It is known that PVA and other such polymers can be precipitated from their water dispersions by dissolved inorganic salts such as Na.sub.2 SO.sub.4 but the precipitated materials contain large quantities of occluded and adsorbed salt and are not readily purifiable to a reusable quality. Also, a precipitation process of this kind involves the use of large quantities of the precipitating salt. Even a very cheap salt could not simply be discharged as waste from the process, but would have to be recovered for reuse because of both economic considerations and the impracticality of dumping large amounts of water-soluble salt which would eventually reach water supplies.